Effects of storage on the estimates of virusmediated bacterial mortality based on observation of preserved seawater samples with TEM

Abstract

There are reports in the literature that the long-term storage (e.g. >30 d) of fixativepreserved seawater samples causes significant decreases in bacterial (BA) and viral abundances (VA). However, the effects of storage on the frequency of visibly infected bacteria (FVIB), and consequently on bacterial mortality due to viral lysis (BMVL), remain to be evaluated. First, to determine the variables that facilitate the prediction of the FVIB value at the time of storage (i.e. FVIBi), we considered a bacterial community composed of 2 groups (i.e. visibly infected bacteria and the others) and assumed an exponential decay relationship for the bacteria in each group during the storage of preserved samples. In the hypothetical model, the FVIBi could be well estimated in terms of BA at the time of storage, a decay rate of BA, and FVIBf and BAf (i.e. FVIB and BA measured at the end of storage, respectively). Further, we tested this idea by applying it to 7 seawater samples that were preserved with 2 % glutaraldehyde (final cone.) and stored at different temperatures for ca. 30 d. For the 3 preserved coastal samples, considerably better estimates of BMVL were obtained by applying the theoretical consideration to estimate the FVIBi (the BMVL was estimated to be 89.9 to 118.7 % of the BMVL at the time of storage [BMVL i], which was the value calculated with FVIBi) than if FVIBf was used (the BMVL was estimated to be 45.5 to 89.9% of the BMVLi value). Interestingly, estimates of the BMVL obtained for the 4 preserved offshore samples were comparable to the FVIB values measured at 3 to 5 d after the start of the experiments, as in the case of preserved coastal samples in the present study, suggesting that it may be possible to estimate FVIBi values for preserved offshore samples. It is recommended that the above-mentioned variables be measured in order to reliably estimate the FVIB value at the time of preservation for fixative-amended and frozen seawater samples that may be stored for a long time, such as oceanic cruises. © Inter-Research 2008.